In a small bowl, whisk all mustard ingredients together. Place in refrigerator until ready to use.
Bring a large pot of water to a boil. Add pierogi and cook stirring occasionally for 6 minutes, drain and set aside.
In a large non-stick pan, heat oil over medium-high heat. Add onion and sauté for 5-6 minutes until lightly golden.
Add kielbasa and sauté for an additional 2-3 minutes. Toss in pierogi and fry for 2-3 minutes more. Season with fresh ground pepper.
Serve warm with hot mustard.